Kinetic Energy/work finding the distance

[razzmatazz]

A 12 kg block is sliding to the left across the floor at a speed of 18 m/s. Opposing the
motion is a 50 N force. How far (in m) will the object travel before its speed is reduced
to 8 m/s?
(a) 21.4
(b) 31.2
(c) 18.4
(d) 7.7
(e) none of these




2. Homework Equations [/b]

lost on how to find distance tho..

K.E. = 1/2mv^2

The Attempt at a Solution

I got d but not sure...i thouhght it was d 7.68 round up
1/2mv2-1/2vi

 

[Shootertrex]

You should be able to use the Work-Energy Theorem to solve this one.

$ƩW=ΔKE$

You know the change in the block's kinetic energy, and you know that friction does negative work. It does not say that there is any other force pushing the block along, so friction is the only force doing work on the block.

Since work is force x distance, this becomes a simple "plug and chug" problem.

 

[cepheid]

Use the work-energy theorem. Since you know the change in kinetic energy, you can find the work done by the 50 N force, and from that, you can find the distance over which that force was applied.